This invention relates to an improved process for the production of thermoplastic polymer mixtures by the reaction of starting components for the preparation of polyurethane in the presence of a previously prepared thermoplast in an extruder, preferably a self-cleaning double screw extruder.
Thermoplastic polyurethanes have been used for many years for a wide variety of applications because of their excellent physical properties. Of the various processes described in the literature for the manufacture of such thermoplasts, the most attractive is the direct production in self-cleaning double screw extruders. These apparatus effect very rapid homogenization of the reaction mixture and the screw shafts clean each other and the cylinders at every point so that the formation of even temporary accretions which could cause decomposition or crosslinking of the products due to prolonged over-heating is avoided.
The processes hitherto known require special measures to bring the reactants together (relatively high molecular weight polyol, chain lengthening agent and diisocyanate) so as to form a homogeneous mixture before polyurethane formation has set in to any significant degree. If the components are mixed too slowly, the polyurethanes obtained are inhomogeneous and contain gel particles which are impossible or difficult to dissolve or melt.
According to the teaching of U.S. Pat. No. 3,233,025, the individual reactants are first brought together in a special mixing apparatus before the homogeneous reaction melt is fed into the screw extruder.
In the process described in U.S. Pat. No. 3,642,964, the reactants must also be mixed very rapidly. To achieve this, the reaction melt is intensively homogenized in a mixing zone of a double shaft screw extruder by means of kneading elements at the latest 6 seconds after the diisocyanate and diol have been introduced. In order to obtain a uniform product, it is also necessary that the melt viscosity should remain virtually uniform over the whole length of the extruder (in the region of about 100 to 1,000 Pas).
According to the teaching in German Offenlegungsschriften 2,302,564 and 2,549,372, however, the reaction mixture must have already been intensively mixed in the extruder with the aid of kneading elements when the reaction melt is still at a low viscosity (about 20 to 70 Pas) if inhomogeneity in the end product is to be avoided.
According to a particular variation of the process disclosed in these German Offenlegungsschriften, fluxes, e.g., thermoplasts, may be added to the product in the extruder during or after the reaction. In such cases, however, the criteria with regard to the kneading zone must also be observed.
In the processes according to German Offenlegungsschriften 2,447,368 and 2,610,980, a decreasing temperature profile must be established in the reaction melt from the intake zone to the point of discharge from the extruder, so that there is little possibility of controlling the polyisocyanate polyaddition reaction. It would appear to be particularly difficult to produce a homogeneous thermoplastic polyurethane according to the teaching of German Offenlegungsschrift 2,610,980 because the starting materials must first be heated to a high temperature and then brought together. Unless the components are mixed very rapidly, a considerable proportion of the polyurethane reaction would presumably take place at a stage when localized over-concentrations of the individual reactants are still present, with the result that a non-uniform product is obtained. Moreover, the initial temperatures are so high that the polyurethane obtained is liable to be damaged by heat.
A process for the production of blends of various thermoplasts in which at least one component is a thermoplastic polyurethane, in extruders, preferably in multishaft screw extruders and most preferably in self-cleaning double shaft screw extruders, has now surprisingly been found, in which the various means mentioned above for achieving rapid mixing of the reaction melt are not required. When carrying out the process according to the invention, there is no need to provide kneading blocks in any particular position of the screw shafts. Moreover, preliminary mixing of the reactants is not necessary, nor is it necessary to maintain a very narrow temperature and/or viscosity profile.